1. Field of the Invention
This invention relates to a digital communication system involving unattended regenerative repeaters and, more particularly, to means for locating a faulty or inoperative one of a plurality of unattended pulse regenerative repeaters which are connected in tandem over a transmission path.
2. Description of the Prior Art
Prior art techniques for testing of tandem regenerative repeaters were directed to those carrier systems such as the Tl which employ return-to-zero bipolar coded pulses. The Tl carrier system employs pulse code modulation in which the digital signal is converted into a bipolar coded signal. A bipolar coded signal is generated from a unipolar (binary) signal by coding binary "0's" into a center level (absence of pulses), and binary "1's" into top or bottom levels in such a manner that every other "1" is inverted. Thus, two successive 1's have opposite polarity as shown below where a unipolar signal is converted to a bipolar code.
Unipolar (Binary): 00011110010100101000111
Bipolar Code: 000+-+-00+0-00+0 -000+-+
A fault-locating test set is used to determine which repeater, in a plurality of repeaters in tandem, is faulty, by sending a special signal which contains both violations of the bipolar code and an audio frequency in its spectrum. This special signal consists of a 3-digit code -- triplets -- generated periodically. This special signal can be regarded as the algebraic sum of two pulse trains: (1) a bipolar coded signal and, (2) a unipolar (binary) signal as follows:
Bipolar Code: +-00000+-00000+-00000+-00000
Unipolar: 00+000000+000000+000000+0000
Special Signal: +-+0000+-+0000+-+0000+-+0000
The unipolar pulse train in the special signal (sum of bipolar and unipolar) represents interference as it causes violations of the bipolar code.
This may be seen by referring again to the special signal shown above. Notice that in the special signal (periodic pulse train) there are always two successive positive pulses +0000+. This is violation of bipolar pattern. These unipolar pulses reduce the crosstalk margin of the bipolar repeater that is designed to pass and regenerate a bipolar coded pulse train. The frequency of occurrence of a triplet may be regulated by the number of 0's which are permitted between repetitions of the triplet. It is apparent that this also changes the density of the special signal. As long as the density of the special signal is low, an operative repeater will accurately reproduce the special signal. Let us now explain what we mean by low density. Reverting back to the special signal, note that the triplets, +-+ (positive, negative, positive pulse) are followed by a string of 0's. The lowest density is when there is one triplet (+-+) per 11 pulse positions; that is each triplet is followed by 8 zeros. Such a density constitutes only small amount of interference. As the density is gradually increased from the 3 (one triplet) out of 11 (total time slots) to 3 out of 10, then 3 out of 9 up to the highest density of 3 out of 4, interference, due to the effect of the unipolar addition to form the special signal, also increases gradually. Thus, the minimum density includes 1 unipolar pulse in 11 time slots, and the maximum density includes 1 unipolar pulse in 4 time slots. At the same time this special signal is switched at an audio rate. This audio frequency corresponds to the frequency assigned to each repeater location. A different audio filter is employed at each repeater location, and the filter is used to extract the sine wave corresponding to the switched audio rate.
As the triplet density is increased, at some point, the repeater under test will start making errors, being unable to reproduce faithfully the triplet pulses. When such errors are made the amplitude of sine wave output of the audio filter, corresponding to repeater location, will be smaller as compared to the amplitude of this sine wave when there are no errors and pulse density is low. The pulse density corresponding to the smaller amplitude of the received sine wave determines repeater margin to noise. A fault-locating test set generates the triplet for transmission, compares the audio tone returned from the repeater to the locally-generated audio tone at the same frequency. The lowest pulse density at which the difference between the locally-generated audio tone and the received audio tone exceeds a predetermined value is the measure of margin. Clearly, repeaters must be tested in the direction of pulse transmission in the order of their location. First, the nearest repeater is tested. If it operates properly, then the next repeater location is selected and so on. For each test, the fault-locating test set is first calibrated. Lowest density (1 out of 11) is sent and the locally generated sine wave is calibrated relative to the received sine wave.
The unipolar spectral density has most of its energy concentrated at low frequencies. Thus the interference is, in effect, low frequency distortion. Also note that in Tl systems, the pulses have a 50% duty cycle. That is the first half of the time slot is +1, -1 or 0, but the second half is always zero.
One such prior art bipolar coded signal testing system is disclosed in U.S. Pat. No. 3,083,270, entitled "Pulse Repeater Marginal Testing System". Here it was explained that the basis of the test signal was a pulse signal of the type normally transmitted over the system. However, this pulse signal did not possess a direct-current component, nor did it possess an additional analog component at a frequency substantially less than the minimum pulse repetition frequency. For the bipolar system a test signal was obtained by superimposing upon a series of bipolar pulses, which were necessary to clock the repeaters, a variable number of unipolar pulses of the same polarity. The variation in the number of the unipolar pulses was used to develop the pulse density requirement for test purposes.
In a second prior art patent, U.S. Pat. No. 3,062,927, entitled "Pulse Repeater Testing Arrangement" unipolar pulses were not used per se. The bipolar pulse pattern consisting of m pulses of one polarity and n pulses of the opposite polarity, m and n being unequal intergers, such that the repetitious patterns have a net direct-current component. The patterns were inverted periodically thereby producing a pulse train having an "identification tone" component at the inversion frequency.
While the repeater test methods disclosed in the prior art permit testing of regenerative repeaters employed in PCM systems which use a bipolar code, the prior art technique is not applicable to a system which uses the modified duobinary code. Therefore, it is a principal object of this invention to provide a test technique which may be employed to locate faulty or inoperative regenerative repeaters for digital systems which use the modified duobinary code.